Add Span to Sampler method params

Also allows us to actually create a PercentageBasedSampler (I
don't think anyone tried it before) without resorting to
lazy beans and proxies.

Another freature added here is a default percentage sampler
if we know that spans need to be exported (zipkin or stream
is present).

Fixes gh-138

spring-cloud-sleuth-zipkin/src/main/java/org/springframework/cloud/sleuth/zipkin/ZipkinAutoConfiguration.java

@@ -18,6 +18,7 @@ package org.springframework.cloud.sleuth.zipkin;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.AutoConfigureBefore;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
@@ -25,7 +26,11 @@ import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.autoconfigure.web.ServerProperties;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.cloud.client.discovery.DiscoveryClient;
+import org.springframework.cloud.sleuth.Sampler;
+import org.springframework.cloud.sleuth.autoconfig.TraceAutoConfiguration;
 import org.springframework.cloud.sleuth.metric.SpanReporterService;
+import org.springframework.cloud.sleuth.sampler.PercentageBasedSampler;
+import org.springframework.cloud.sleuth.sampler.SamplerConfiguration;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
@@ -34,21 +39,22 @@ import org.springframework.context.annotation.Configuration;
  * @author Spencer Gibb
  */
 @Configuration
-@EnableConfigurationProperties
+@EnableConfigurationProperties({ZipkinProperties.class, SamplerConfiguration.class})
 @ConditionalOnProperty(value = "spring.zipkin.enabled", matchIfMissing = true)
+@AutoConfigureBefore(TraceAutoConfiguration.class)
 public class ZipkinAutoConfiguration {
 
 	@Bean
 	@ConditionalOnMissingBean(ZipkinSpanReporter.class)
-	public ZipkinSpanReporter reporter(SpanReporterService spanReporterService) {
-		ZipkinProperties zipkin = zipkinProperties();
+	public ZipkinSpanReporter reporter(SpanReporterService spanReporterService, ZipkinProperties zipkin) {
 		return new HttpZipkinSpanReporter(zipkin.getBaseUrl(), zipkin.getFlushInterval(),
 				spanReporterService);
 	}
 
 	@Bean
-	public ZipkinProperties zipkinProperties() {
-		return new ZipkinProperties();
+	@ConditionalOnMissingBean
+	public Sampler defaultTraceSampler(SamplerConfiguration config) {
+		return new PercentageBasedSampler(config);
 	}
 
 	@Bean

spring-cloud-sleuth-zipkin/src/main/java/org/springframework/cloud/sleuth/zipkin/ZipkinProperties.java

@@ -26,8 +26,6 @@ import lombok.Data;
 @ConfigurationProperties("spring.zipkin")
 @Data
 public class ZipkinProperties {
-	// Sample rate = 1.0 means 100% of requests will get traced.
-	private float fixedSampleRate = 1.0f;
 	/** URL of the zipkin query server instance. */
 	private String baseUrl = "http://localhost:9411/";
 	private boolean enabled = true;